Use `iretq` method of jumping to 32-bit so VirtualBox (and maybe QEMU/KVM too?)
can cope and not get its `ljmp <mem48>` emulation bug(s) tickled.

@@ -1,9 +1,9 @@
 /*
  * Parts copyright Michael Brown <mbrown@fensystems.co.uk>
  *
- * Copyright (c) 2019, Joyent, Inc.
+ * Copyright 2020 Joyent, Inc.
  */
 
 FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL )
 
 /* CR0: protection enabled */

@@ -60,11 +60,18 @@
 
         leaq    entry_gdtr(%rip), %rax
         lgdt    (%rax)
 
         /* Load our new %cs. */
-        ljmp    *newcs_vector
+        movq    %rsp, %rax
+        pushq   $GDTSEL_DATA
+        pushq   %rax
+        pushf
+        pushq   $GDTSEL_CODE
+        lea     newcs(%rip), %rax
+        pushq   %rax
+        iretq
 
         .code32
 newcs:
 
         movw    $GDTSEL_DATA, %ax

@@ -106,13 +113,10 @@
         movq    %rsi, %rsp
         jmp     *%rdx
 
         .data
 
-newcs_vector:
-        .long   newcs, GDTSEL_CODE
-
         .align 16
 entry_gdt:
         /* null entry */
         .word   0x0, 0x0
         .byte   0x0, 0x0, 0x0, 0x0